1. Field of the Invention
The subject invention relates to light emitting diodes mounted in an array on a circuit board and, more particularly, to the dissipation of heat generated by the diodes. Such light emitting diode assemblies have particular utility in traffic signals.
2. Description of the Prior Art
Newly developed light emitting diode traffic signals are exhibiting useful service lives that are much shorter than predicted. The expected life of such signals is at least five to seven years, but actual field experience with some of these devices demonstrates significant dimming of the L.E.D. after two years or less. The relatively high cost of the L.E.D. conversions suggests that a minimum three year life is necessary in order to amortize the acquisition cost. Energy savings, which can exceed eighty percent, are typically used to finance the conversion of incandescent lamps to the L.E.D. variants, in combination with the cost savings attributed to lower relamping expenses.
Both the purchasers and manufacturers of L.E.D. traffic signals seem to be puzzled by the relatively poor performance of the devices in the field. It is important to note that most L.E.D. component manufacturers predict lifetimes in excess of one hundred thousand hours. The widely divergent results observed in actual field experience vis a vis that observed in a laboratory environment suggests that the field operating conditions are very unlike those used to predict device lifetimes.
L.E.D.s are commonly rated at some nominal average operating current (20 mA) and temperature (typically 25.degree. C.) for a given luminous output. Useful life is specified as the point in time (hours) at which the luminous output is half the initial value.
Recent experiments with a wide variety of L.E.D.s suggest an exponential relationship of life versus operating temperature. The well known Arhenius function is an approximate model for LED degradation: D.varies.te.sup.kT where D is the degradation, t is time, e the base of natural logarithms, k an activation constant and T the absolute temperature in degrees Kelvin.
While this formulation is necessarily inexact, and is clearly device dependent, within a given L.E.D. family the empirical data can be modeled satisfactorily. The impact of this realization is dramatic. While room temperature (25.degree. C.) lifetimes may in fact approach one hundred thousand hours, operation at close to 90.degree. C. may reduce L.E.D. life to less than seven thousand hours.
Interestingly, neither users nor manufacturers of L.E.D.s specify operation of L.E.D. devices at temperatures approaching 90.degree. C. Actual data collected in solar heating studies of traffic signals show that internal temperatures approaching 85.degree. C. may be rather common in the U.S. Southwest. In fact, at ambient air temperatures of 40.degree. C. which are rather common, solar gain within the traffic signal housing can further increase the temperature nearly 30.degree. C., without even operating the L.E.D. signals. The added thermal loading due to power supply losses and L.E.D. dissipation pushes the actual L.E.D. operating environment to temperatures in excess of 85.degree. C. (185.degree. F.) for at least a significant part of the day.
The problem of high internal temperatures is exacerbated in cases where L.E.D. lamps are intermixed with incandescent lamps. For example, if, in a given housing, only the red incandescent lamp is converted to an L.E.D. lamp, and the amber and green incandescent lamps are retained, the heat generated by the illuminated incandescent lamps will greatly increase the temperature surrounding the L.E.D. lamp.
Obviously, venting the L.E.D. traffic lamp assembly or module into the sealed traffic signal housing, is futile. Rejecting heat into an environment of higher temperature than that of the source is thermodynamically impossible. The key to improving the life of the L.E.D.s in traffic signal service is to reduce the temperature of the L.E.D. environment. Note that little can be done to modify the "ambient" temperature which is the normal surrounding air temperature.
U.S. Pat. No. 4,729,076 to Masami et al strives to lower the temperature of the LED array by attaching a finned heat sink assembly. However, there is an impediment or restrictor in the path of the heat from the light emitting diodes to the heat sink; to wit, a resin filler or adhesive which is a very poor heat conductor. The Masami '076 patent recognizes the problem of positioning the heat sink within the traffic signal housing where it must exchange heat with the air within the housing. As noted in the Masami '076 patent, some means of ventilation must be provided by vents, louvers, fans or the like, when the heat sink is within the housing. Such provisions are not particularly effective in hot climates, and they subject the signal to dirt and moisture infiltration. Since the lens, reflector and lamp assembly is not separately sealed in traditional housings, moisture or dust that enters a conventional signal housing may degrade the optical performance of the unit and cause corrosion of exposed electrical components. Experiments with baffled vents suggest that substantial (size) openings at the bottom and top of the traffic signal housing would be necessary to provide unimpeded (low back pressure) air flow through the housing. Clearly, retrofitting LED lamps into existing traffic signal housings that do not have venting provisions is futile, because of heat loading. Field modification of existing housings to provide adequate venting is costly, time consuming and of limited utility because of the size of openings that are necessary. To overcome this venting problem, the Masami '076 patent suggests placing the heat sink on the back of the housing and exposed to ambient air with a heat conduit to transfer heat from the LEDs to the heat sink. Again, field modification of existing housings to mount a heat sink on the back of the housing is costly, time consuming and of limited utility because there still remains the restriction or impediment to heat transfer from the LEDs to the heat sink. Furthermore, heating of the heat sink by solar radiation would greatly reduce its effectiveness.
The luminous intensity of LEDs is a strong function of their temperature. Typically the highest performance LEDs available today, Al In Ga P devices, show a 0.9% per .degree. C. decrease in luminous output.
The amount of self generated heat in a large array of LEDs can be substantial. For example, a red traffic signal containing 300 discrete LEDs on a conventional circuit board can be expected to exhibit a temperature rise of over 30.degree. C. in continuous operation. The resulting diminution in luminous output of nearly 30% is significant; when coupled with higher ambient temperatures that may be encountered in certain venues, this self generated heat can be crippling . . . reducing the luminous intensity of the signal to unsafe levels.
Several prior art patents address the question of heat extraction from an LED array, but an essential parameter in the successful implementation of this art has eluded previous investigators. Many technical issues must be carefully considered in the design of reliable LED signals, but among the most important are the thermal properties of the various components that form the heat flow path.
The tremendous disparity in thermal conductivity among materials used in electronic construction is instructive and is shown in the following table:
______________________________________ THERMAL CONDUCTIVITY TABLE Material (K) ______________________________________ 1. Still Air 0.2 2. Epoxy Resin (unfilled) 1.0 3. Polyester Film 1.1 4. Mica 3.7 5. Thermally Conductive (filled) 10.7 Epoxy (Electrically Insulating) 6. Silver Conductive Epoxy 12.0 7. Steatite 19.1 8. Alumina 237 9. Alloy Steel 415 10. Aluminum 1700 11. Copper 2500 ______________________________________ *Thermal conductivity (K) in BTU/hr .multidot. ft.sup.2 - .degree. F./in
The interaction of the thermal properties of some of these materials, in LED arrays, indicates that prior approaches to the problem were not well conceived nor well understood. In fact, studies of existing hardware that embody the technology described by the prior art, show that at least an order of magnitude improvement in performance is attainable by the application of the methods and apparatus of the present invention.
Typically, the prior art, as exemplified by Roney, et al. in U.S. Pat. Nos. 5,528,474 and 5,632,551, comprises an LED circuit board potted or encapsulated in a filled resin matrix within a metal shell. The intended purpose of the filled resinous encapsulant, which may be a thermally conductive epoxy, is to conduct heat from the LED array into the metal housing which acts as a heat dissipator.
The improvement in thermal performance of LED devices that embody the technology taught by Roney, et al. over the apparatus disclosed by Masami, et al. in U.S. Pat. No. 4,729,076 is substantial.
Masami's use of unfilled resin and a non thermally coupled insulation sheet greatly diminishes the flow of heat from the LED array to the heat dissipator.